Fluorine-Induced Rigidity and Entropy Effects in Mixed-Halide 2,2-Difluoroethylammonium Cadmium Hybrids

We report a comprehensive study of structure-related properties in difluorinated 2D cadmium halides (F₂EA)₂CdX₄ (X = Cl^-, Br^-, and mixed Br/Cl ratio of 60:40). Structural single-crystal X-ray diffraction studies demonstrated that the halide substitution introduces positional disorder, affecting the organic cations and the inorganic layers of corner-sharing octahedra. Differential scanning calorimetry showed large entropy changes related to the phase transition, largely exceeding the values expected for the order-disorder processes, indicating a complex mechanism involving the ordering of cations, hydrogen bond reorganization, and distortion of inorganic sublattices. Structural studies, together with IR spectroscopy experiments supported by DFT calculations, demonstrated that fluorination enhances the intermolecular F···F interactions, leading to unusually short contacts and increasing the rigidity of the hybrid. The noncovalent interaction calculations confirm that the short F···F contacts are not associated with stabilizing interactions but rather with sterically enforced packing. The phase transition is accompanied by significant changes in molecular dynamics and the tilting of inorganic slabs. Optical studies indicated that tunable emission is governed by a halide substitution and a change in the structural rigidity resulting from hydrogen bonds, accompanied by the F···F interactions. These findings highlight the important role of multiple fluorination and halide mixing in controlling structural disorder, lattice dynamics, and optoelectronic response in layered hybrid crystals.